It is becoming increasingly common to equip vehicles with so-called airbag restraints for both the driver and the front seat occupant. Since it can be assumed that the driver's seat of a vehicle is always occupied when the vehicle is in motion, it is generally desirable that the airbag on the driver's side be deployed if the vehicle is subjected to a deceleration above a predetermined threshold. Frequently, though, the driver is alone in the vehicle and, consequently, deployment of the passenger side airbag is unnecessary. Since unnecessary deployment of the passenger side airbag implies increased repair costs to refit the airbag, there is a demand from i.a. insurance companies to provide vehicles equipped with twin airbags with means to disarm the passenger side airbag when the passenger seat is unoccupied.
Clearly, there are many various ways of establishing whether a vehicle seat is occupied. For instance, the vehicle may be provided with a simple on-off switch which is activated manually by the driver or passenger. Such a system is however unfavoured since the driver or passenger may forget to arm the airbag actuator when the seat is occupied. Accordingly, a means of detection which functions without human intervention is sought.
From U.S. Pat. No. 3,863,209 there is known a vehicle seat having a load-dependent switch arranged in the seat cushion. When the seat is occupied by a person of sufficient weight, the switch is activated to arm the airbag sensor. A disadvantage with such a system is that the load-dependent switch cannot distinguish between a human being and a heavy item which may be placed on the seat. Furthermore, it is generally undesirable for an airbag to be deployed if the upper torso and head of the seat occupant are too close to the airbag before it is deployed. The use of a load dependent switch in the seat cushion clearly cannot solve this problem.
It is also known to use optical or ultrasonic transmitters and sensors to detect the presence of a seat occupant, though such systems are relatively difficult to install, requiring components to be mounted at different locations within the vehicle cabin. An example of such a system is disclosed in DE-A-4 023 109.
One method of detection to which much time is being devoted employs a seat electrode which, together with the vehicle body as a counter-electrode, constitutes a capacitor. The capacitor is connected to an oscillating circuit, the frequency of which depends on the capacitance between the electrodes of the capacitor. The capacitance is dependent on the dielectric constant of the material which is present between the electrodes. Thus, when the seat is unoccupied, a low dielectric constant exists, thereby providing low capacitance. This implies that the oscillator circuit oscillates at a relatively high frequency. Conversely, when the seat is occupied by a passenger, a higher dielectric constant is present and consequently the oscillator circuit oscillates at a relatively low frequency. By providing a control circuit which is activated by the presence of a frequency of certain magnitude, an arming signal can be transmitted to the airbag sensor when the seat is occupied. A system of this type is described in DE-A-4 110 702.
Due to the risks involved by deployment of an airbag when the passenger seat is occupied by a rear-facing child seat, it is important that the chosen detection system can distinguish between a child occupying the passenger seat and sitting in a conventional position and a child sitting in a rear-facing child seat mounted on the front passenger seat. A problem which has heretofore existed with known capacitance-dependent systems is that the capacitance between the electrodes is significantly increased if a body with a high dielectric constant comes between the electrodes. This can occur, for example, if the passenger seat is occupied by a child in a child seat and an adult passenger contacts the child or the seat. Such an action causes the frequency in the oscillator circuit to decrease to a level which could well be below the predetermined disarming frequency for the airbag sensor, thereby providing the sensor with false information. Thus, although the passenger seat is occupied by a child seat, it is possible that if the vehicle were to be involved in an accident which would deploy the driver's side airbag, the passenger side airbag could also be deployed.